The present invention generally relates to a magnetometer and more specifically relates to an electronic compass for use in a vehicle.
Magnetometers are used in many different applications. One such application is an electronic compass for a vehicle. In such electronic compasses, magnetometers are utilized to ascertain the vehicle heading relative to the Earth""s magnetic north pole. A typical electronic compass includes two magnetic field sensors both disposed with their axes lying in a horizontal plane with a first sensor having its axis aligned in parallel with the longitudinal axis of the vehicle and the second sensor having its axis disposed orthogonal to the axis of the first sensor. The sensors are then utilized to detect the magnitude of orthogonal, horizontal, axially aligned components of the Earth""s magnetic field vector such that a processing circuit may then compute the heading of the vehicle relative to the Earth""s magnetic field vector.
Several different forms of magnetometers have been utilized for use in vehicle electronic compasses. Examples of some of these types of magnetometers include those utilizing flux-gate sensors, magneto-resistive sensors, and magneto-inductive sensors. Magneto-inductive sensors may be configured in different forms including L/R sensors and LC sensors. In both these forms of magneto-inductive sensors, a coil is wound around a core material. The sensor has a characteristic that its inductance varies linearly in response to a magnetic field, but only throughout two predetermined ranges of values of the external magnetic field. By viewing a plot of the sensor inductance versus the magnetic field strength (see FIG. 5, for example), one can see that the resultant curve is substantially symmetric about the point at which the magnetic field strength is zero. Accordingly, it is commonplace to apply a bias current to the sensor coil such that an artificial magnetic field is generated about the core material. The artificially generated magnetic field produced by this bias current is summed with the external magnetic field. External magnetic fields that are in the same direction as the artificial magnetic field generated by the bias current add to one another while an external magnetic field in the opposite direction of the artificial magnetic field is subtracted from the artificial magnetic field. Thus, by measuring the change in inductance of the sensor, the strength of the axially aligned magnetic field component may be ascertained.
To measure the inductance change of the sensor, circuit configurations where the responding frequency changes with changing sensor inductances have been employed. With such circuits, the changes in inductance of the sensor produces approximately proportional changes in the frequency of the signal output from the sensor. The frequency change may then be measured to determine the strength of the external magnetic field.
A problem encountered in such magnetometers is that the core material characteristics vary with temperature and age. One solution to this problem is disclosed in European Patent No. 0045509 B1. This European patent discloses that the bias current polarity on the sensor coil may be reversed with measurements taken with the bias current at both polarities such that the difference between the two measurements corresponds to the external magnetic field. The measurement thus taken is independent of any variance of the core material caused by temperature variation or age.
U.S. Pat. No. 5,239,264 discloses a similar technique. FIGS. 1 and 2 of this application correspond to FIGS. 3 and 4 of the ""264 patent. As shown in FIGS. 1 and 2, the permeability function u(H) of the core material varies as a function of the strength of the magnetic field H over a particular range of the magnetic field strength. As apparent from this graph, there are generally two regions of the curve in which the permeability varies with respect to the change in magnetic field strength. One of these regions has a positive slope whereas the other region has a negative slope. In the ""264 patent, the polarity of the DC bias current is alternatingly reversed so as to provide readings at both polarities. The two readings may then be subtracted from one another to arrive at the magnetic field strength of the component of the Earth""s magnetic field sensed by that particular sensor coil.
In both the above-mentioned U.S. Pat. No. 5,239,264 and published European Patent No. 0045509 B1, the DC bias current remains at a constant level and only the polarity of the bias current is reversed. One problem with providing an electronic compass in an automobile is that the automobile may distort the external magnetic field. Further, as the vehicle travels past objects such as bridges, subways, power lines, railroad tracks, and other objects, these objects may cause disturbances in the magnetic field that are sensed by the electronic compass. Such magnetic field disturbances may produce magnetic fields that cause the magnetic field sensed by the sensor coils to fall within a non-linear region of the inductance versus magnetic field strength curve. Thus, the magnetometers of the above-described patents have limited ranges in which they can accurately detect the strength of the external magnetic field.
Accordingly, there exists a need for an electronic compass having the ability to accurately sense magnetic field components throughout a greater range than is presently provided by conventional magnetometers.
According to one embodiment of the present invention, a magnetometer is provided that comprises a sensor for sensing a magnetic field, a biasing circuit, and a processor. The sensor generates an output signal having a signal characteristic that varies in response to the sensed magnetic field and in response to an applied bias. The biasing circuit dynamically biases the sensor in response to a bias setting signal. The processor is coupled to receive the output signal from the sensor and coupled to the biasing circuit. The processor is operable to generate the bias setting signal and thereby control the biasing circuit to dynamically bias the sensor such that the signal characteristic of the output signal is maintained within a relatively small target range of levels. The processor determines the magnetic field component sensed by the sensor as a function of the bias setting applied to the sensor.
According to another embodiment of the present invention, a magnetometer is provided that comprises a first sensor for sensing a first component of a magnetic field, a second sensor for sensing a second component of the magnetic field, a biasing circuit, and a processor. Each of the sensors generates an output signal having a frequency that varies in response to the sensed component magnetic field and in response to an applied bias current. The biasing circuit generates bias currents to dynamically bias the first and second sensors. The processor is coupled to receive the output signals from the sensors and is coupled to the biasing circuit. The processor is operable to control the biasing circuit to dynamically vary the bias currents applied to the sensors such that the frequency of the output signals is maintained within one or more target frequency ranges. The processor determines the magnetic field components sensed by the sensors as a function of the biasing currents applied to the sensors.
According to another embodiment, an electronic compass for a vehicle is provided that comprises a first magnetic field sensor for sensing a first component of a magnetic field, a second magnetic field sensor for sensing a second component of the magnetic field that is orthogonal to the first component, a biasing circuit, a processing circuit, and a heading indicator coupled to the processing circuit for indicating the vehicle heading. Each of the sensors generates an output signal having a signal characteristic that varies in response to both the sensed component magnetic field and in response to an applied bias current. The biasing circuit generates bias currents to dynamically bias the first and second sensors. The processing circuit is coupled to receive the output signals from the sensors and is coupled to the biasing circuit. The processor is operable to control the biasing circuit to dynamically vary the bias currents applied to the sensors such that the signal characteristics of the output signals are maintained within one or more target ranges. The processing circuit computes a vehicle heading as a function of the biasing currents applied to the sensors.
According to another embodiment, a method of determining the strength of a magnetic field component comprises the steps of: providing a magnetic field sensor that generates an output signal having a signal characteristic that varies in response to the strength of a sensed magnetic field component and in response to an applied bias setting; dynamically varying a bias setting of the sensor such that the signal characteristic of the output signal is maintained within a target range; and determining the strength of the sensed magnetic field component as a function of the bias setting of the sensor.
According to another embodiment, a magnetometer is provided that comprises a sensor for sensing a magnetic field component, a magnetic field generating mechanism, and a processor coupled to receive the output signal from the sensor and coupled to the magnetic field generating mechanism. The sensor generates an output signal having a characteristic that varies substantially linearly in response to the sensed magnetic field components throughout a first range of magnetic field levels. The magnetic field component varies throughout a second range of magnetic field levels. The magnetic field generating mechanism generates a magnetic field that is summed with any external magnetic field such that the resultant magnetic field is sensed by the sensor. The strength of the generated magnetic field is selectively variable. The processor is operable to control the magnetic field generating mechanism to select the field strength of the generated magnetic field and thereby dynamically shift and/or maintain the second range within the first range. The processor is further operable to determine the magnetic field component sensed by the sensor in response to the output signal received from the sensor.
According to another embodiment, a magnetometer is provided that comprises a sensing element having a sensor characteristic that varies in response to a magnetic field, and an amplifier having an input for receiving a driving signal. The sensing element is coupled within a feedback loop of the amplifier. The amplifier generates an output signal having a signal characteristic that varies at least partially in response to variance in the sensor characteristic.
According to another embodiment, a magnetometer comprises a first sensing element having a sensor characteristic that varies in response to a magnetic field; a second sensing element having a sensor characteristic that varies in response to a magnetic field; a single first analog switch provided for selecting the first sensing element; a single second analog switch provided for selecting the second sensing element; and a processor coupled to receive output signals from a selected one of the first and second sensing elements and coupled to the first and second analog switches to select one of the first and second sensing elements.
According to another embodiment, a magnetometer comprises a sensor for sensing a magnetic field, the sensor generating an output signal having a signal characteristic that varies in response to the sensed magnetic field and in response to an applied bias; first and second high gain amplifiers each having an input, one of the amplifiers being coupled to the sensor; a biasing circuit for biasing the sensor, the biasing circuit being coupled between the inputs of the first and second high gain amplifiers; and a processor coupled to receive the output signal from the sensor, the processor determines the magnetic field component sensed by the sensor.
According to another embodiment, a magnetometer comprises a resonant sensor for sensing a magnetic field, the sensor generating an output signal having a signal characteristic that varies in response to the sensed magnetic field and in response to an applied bias; a biasing circuit for adjustably biasing the sensor at two or more bias levels for each bias polarity; and a processor coupled to receive the output signal from the sensor, the processor determines the magnetic field component sensed by the sensor, wherein the peak to peak excursion of the magnetic field level in the resonant sensor during a resonant cycle is a fraction of the field level excursion range due to the adjustment of the bias circuit over its total range of adjustment.
According to another embodiment, a magnetometer comprises a resonant sensor for sensing a magnetic field, the sensor generating an output signal having a signal characteristic that varies in response to the sensed magnetic field; and a processor coupled to receive the output signal from the sensor, the processor determines the magnetic field component sensed by the sensor, wherein the peak to peak excursion of the magnetic field level in the resonant sensor during a resonant cycle is less than the total range of the magnetic field to be measured.
According to another embodiment, a magnetometer comprises a resonant sensor for sensing a magnetic field, the sensor generating an output signal having a signal characteristic that varies in response to the sensed magnetic field; an excitation circuit coupled to the resonant sensor for supplying an excitation signal thereto, the excitation circuit limits the amplitude of the excitation signal to prevent significant saturation of the response of the resonant sensor to the excitation signal; and a processor coupled to receive the output signal from the sensor, the processor determines the magnetic field component sensed by the sensor.
According to another embodiment, a magnetometer comprises a sensor for sensing a magnetic field, the sensor generating an output signal having a signal characteristic that varies in response to the sensed magnetic field and in response to an applied bias; a biasing circuit for adjustably biasing the sensor at two or more bias levels; and a processor coupled to receive the output signal from the sensor, the processor determines the magnetic field component sensed by the sensor as a function of the signal characteristic of the output signal from the sensor and as a function of a slope of the output signal versus bias level.
According to another embodiment, a magnetometer comprises a sensor for sensing a magnetic field, the sensor generating an output signal having a signal characteristic that varies in response to the sensed magnetic field and in response to an applied bias; a biasing circuit for adjustably biasing the sensor to at least a first bias level and a second bias level; and a processor coupled to receive the output signal from the sensor, the processor determines the magnetic field component sensed by the sensor as a function of an average of the output signal level when at the first and second bias levels.
According to another embodiment, a magnetometer comprises a first sensing element having a sensor characteristic that varies in response to a magnetic field; a second sensing element having a sensor characteristic that varies in response to a magnetic field; a biasing circuit for adjustably biasing the first sensing element to at least a first bias level and a second bias level, and for adjustably biasing the second sensing element to at least a third bias level and a fourth bias level; and a processor coupled to the biasing circuit and to the first and second sensing elements to receive the output signals from the sensing elements. The processor measures the magnetic field components sensed by the sensing elements by sequentially: sampling the output signal of the first sensing element at the first bias level, sampling the output signal of the second sensing element at the third bias level, sampling the output signal of the second sensing element at the fourth bias level, sampling the output signal of the first sensing element at the second bias level, determining the magnetic field component of the first sensing element as a function of the samples taken at the first and second bias levels, and determining the magnetic field component of the second sensing element as a function of the samples taken at the third and fourth bias levels.
According to another embodiment, a magnetometer comprises a first sensing element having a sensor characteristic that varies in response to a magnetic field; a second sensing element having a sensor characteristic that varies in response to a magnetic field; at least one analog switch provided for selecting the first or second sensing element, the at least one analog switch having a resistance; a biasing circuit for supplying a biasing current to a selected one of the sensing elements; and a processor coupled to receive output signals from a selected one of the first and second sensing elements and coupled to the at least one analog switch to select one of the first and second sensing elements, the processor determines the magnetic field components sensed by the sensing elements, wherein the biasing circuit is configured to supply a biasing current that is substantially independent of the resistance of the at least one analog switch.
According to another embodiment, a magnetometer comprises a first sensing element having a sensor characteristic that varies in response to a magnetic field; a second sensing element having a sensor characteristic that varies in response to a magnetic field; at least one analog switch provided for selecting the first or second sensing; a biasing circuit for adjustably biasing the sensing elements to at least a first bias level and a second bias level; and a processor coupled to receive output signals from a selected one of the first and second sensing elements and coupled to the at least one analog switch to select one of the first and second sensing elements, the processor determines the magnetic field components sensed by the sensing elements, wherein the biasing circuit biases one of the sensing elements at the first bias level and subsequently biases the same sensing element at the second bias level without any analog switch changing states.
According to another embodiment, a magnetometer comprises a sensor for sensing a magnetic field, the sensor generating an output signal having a signal characteristic that varies in response to the sensed magnetic field and in response to an applied bias; a biasing circuit for adjustably biasing the sensor, the biasing circuit including a digital-to-analog converter; and a processing circuit including a readout device coupled to receive the output signal from the sensor, the processing circuit measures the magnetic field component sensed by the sensor by taking at least one reading of the output signal from the sensor, wherein the resolution in reading the output signal is a function of both the digital-to-analog converter and the readout device.
According to another embodiment, a method of making a plurality of magnetic field sensing inductors comprises the sequentially performed steps of: providing a core for each field sensing inductor; testing the core of each field sensing inductor; and winding a coil around each core, the number of windings about each core being adjusted based on the results of testing of the core.
According to another embodiment, a magnetometer comprises a resonant sensor for sensing a magnetic field, the sensor generating an output signal having a signal characteristic that varies in response to the sensed magnetic field; an excitation circuit coupled to the resonant sensor for supplying an excitation signal thereto having an AC component; a filter for filtering the excitation signal prior to application to the resonant sensor, the filter making the excitation signal approximately sinusoidal; and a processor coupled to receive the output signal from the sensor, the processor determines the magnetic field component sensed by the sensor.
According to another embodiment, a magnetometer comprises a sensor for sensing a magnetic field, the sensor generating an output signal having a signal characteristic that varies in response to the sensed magnetic field and in response to an applied bias current; a biasing circuit for adjusting a bias current supplied to the sensor in response to a bias setting; and a processor coupled to receive the output signal from the sensor and coupled to the biasing circuit for supplying the bias setting, the processor determines the magnetic field component sensed by the sensor as a function of the bias setting, wherein the bias setting selected to determine the magnetic field component is based on the difference in bias current at two points for which the output signal achieves a target response.
According to another embodiment, a magnetometer comprises a sensor for sensing a magnetic field, the sensor generating an output signal having a signal characteristic that varies in response to the sensed magnetic field and in response to an applied bias current; a biasing circuit for adjusting a bias current applied to the sensor in response to a bias setting; and a processor coupled to receive the output signal from the sensor and coupled to the biasing circuit for supplying the bias setting, the processor determines the magnetic field component sensed by the sensor as a function of the bias setting, wherein the bias setting selected to determine the magnetic field component is based on no more than five prior raw readings obtained from the sensor.
These and other features, advantages, and objects of the present invention will be further understood and appreciated by those skilled in the art by reference to the following specification, claims, and appended drawings.